1. Field of the Invention
The invention is a method and apparatus for characterizing reflected ultrasonic pulses and more particularly to such method and apparatus for characterizing the amplitude of ultrasonic pulses reflected from discontinuities in living tissue including those resulting naturally and from trauma such as burns, wounds, scars, and surgery as well as in other ultrasonic sound wave propagation mediums in which the attenuation coefficient and velocity of reflected ultrasonic pulses remains substantially unchanged.
2. Description of the Prior Art
Generally, there are three types of so called degrees of burn wounds in human tissue--first, second, and third which are related to severity, diagnosis, and treatment. Briefly, first degree burns involve only the outer skin or epidermis, second degree burns extend into and through the dermis and third degree burns extend through both the epidermis and dermis into the subcutaneous layer of fat of the body.
It is generally believed that the respective layers of human tissue, the epidermis, dermis, and underlying layer of subcutaneous fat have different and detectable acoustical impedances which form interfaces or discontinuities therebetween that reflect ultrasonic sound waves and which layers are respectively altered when subjected to different degrees of burning or other trauma.
For example, see U.S. Pat. No. 4,080,960, dated Mar. 28, 1978, which discloses an ultrasonic echo method of determining burn depths by sensing an acoustic pulse reflected by the interface between burned and unburned human tissue. Also see U.S. Pat. No. 4,733,668, dated Mar. 29, 1988, which discloses an ultrasonic method and apparatus for examining human tissue including linear time gain amplification to compensate for differences in the attenuation of ultrasound energy in passing through human tissues having different attenuation characteristics.
Ultrasonic diagnostic instruments are generally known that compensate for the change of attenuation of an ultrasonic pulse reflected from a discontinuity detected in living human tissue by employing linear time gain circuitry as described in U.S. Pat. No. 4,733,668. It is also a known scientific principle that the attenuation of an ultrasonic pulse in passing through a given propagation medium is exponential. Ultrasonic pulse-echo apparatus including exponential time gain compensation has been generally proposed to compensate for such attenuation. For instance, see U.S. Pat. No. 4,043,181 dated Aug. 23, 1977.
With or without the gain compensation of the prior art, techniques to detect ultrasonic reflections in human tissue and visually display them on a suitable oscilloscope with accuracy and resolution for diagnostic purposes is difficult because of variations in attenuation introduced by aging, color, composition, sex, and the many discontinuities or interfaces in human tissue that also substantially change the attenuation as well as velocity of the reflected pulses.
The latter produces a clutter of reflected pulses all of which are subjected to the particular time gain compensation, if any, prior to being displayed in typically an A-scan. The value or size of the amplitude of the respective reflected pulses is meaningless in conventional systems. Typically, the clutter of reflected pulses in such A-scans makes it difficult to clearly identify and accurately locate reflected pulses of interest on the X or time axis, except by a skilled interpreter. Thus, even knowing the pulse velocity, conventional ultrasonic echo measurement and display systems do not accurately measure and display the distance of the discontinuity causing the reflection from the source of ultrasonic pulses-a critical determination is burn depth monitoring or scanning.
Thus, the type, linear or exponential, of prior art gain compensation used in processing the reflected pulses in conventional ultrasonic pulse echo diagnostic methods and circuitry is not used to characterize the amplitude of selected reflected pulses to accurately determine their point of origin within a propagation medium such as human tissue.